This invention relates to optical systems capable of providing a magnification which can be varied.
Optical systems which can provide a variable magnification are often required and have been known for a long time. In the visible region of the electromagnetic spectrum, variable magnification optical systems have been implemented for many years using a series of lenses whose separations can be varied in order to change the magnification provided by the lens system. A problem with lens systems is that because of their refractive nature chromatic aberrations can occur and this type of lens system often requires a number of extra elements in order to overcome this problem. At wavelengths outside the visible region, for example in the infrared region, the number of materials which have the necessary transmission qualities to form such lenses is severely restricted and this restricted choice of material can have the effect of reducing the performance of the optical system that can be achieved over a range of wavelengths. Also the cost of the system is increased.
This problem has been dealt with in the past by constructing variable magnification optical systems entirely of mirrors. An advantage of a mirror system is that there is no chromatic abberation. However a disadvantage of many mirror systems is that each mirror generally works off axis and as a consequence aberrations occur and increase rapidly as the light propagates through the system of mirrors. Another disadvantage of many mirror systems is the complexity of the mechanical system that is required in order to move the individual mirrors in order to achieve a change in magnification. This arises because the mirrors are not positioned and moved on a common axis as in the case of lenses and the position of the mirrors has to be controlled in two rather than one dimension.
The present invention is concerned with an optical system which has been developed in order to deal with both the aberration and mechanical articulation problems associated with the prior art.
According to the present invention there is provided an optical system capable of providing a magnification which can be varied comprising a pair of ellipsoidal mirrors which are so mounted and coupled that the point at which radiation reflected from the first mirror is focused corresponds substantially with the input focus of the second mirror. The mirrors may be mounted for pivotable movement so that the angle between their axes can be varied.
The optical system may include a first pair of ellipsoidal mirrors and a second pair of ellipsoidal mirrors, said pairs of mirrors being so mounted and coupled that the output focus of the first pair corresponds substantially to the input focus of the second pair. The mirrors may be coupled by articulation means and the articulation between the output mirror of the first pair and the input mirror of the second pair is at the output focus of the first pair. The said point of articulation may be constrained to move along a predetermined path when the mirrors are moved in order to achieve a change in magnification.